This invention relates to an endoscope using a solid state image pick-up device, wherein the dynamic range of the signals obtained by said device is controlled by both the illuminating means and a gain-variable amplifying means. The control by the illuminating means is supplemented by the gain-variable amplifying means.
Recently, various endoscopes using a solid stare image pick-up device such as CCD (charge coupled device) have been proposed. In such endoscopes, when the color imaging method uses a single CCD, a method of providing a color filter separately or in one piece with the CCD and a method to sequentially switch the light of 3 primary colors and project it onto the CCD have been contemplated. The detailed configurations of such methods are described, for example, in Japan Parent Journal Nos. 55-54933 and 51-65952.
A CCD includes many photosensitive picture elements called picture cells as described in an article in "Scientific American" published in February 1974. The picture cells are arranged perpendicular to the wide angle center latitude of the object lens system to form a rectangular grid pattern as a whole. If such grid pattern surface is used as a light receiving surface and light energy is incident on the light receiving surface, the electrons existing within each picture cell region are accumulated or gathered to form a lump of charges. After a specific time, these charges are moved to an electrode corresponding to each picture cell and an output signal proportional to the size of the charge lump is produced. Therefore, the lump of charges for part of a received image with low brightness is small, and that for a part of the image with a high brightness is large.
The sensitivity and dynamic range of the CCD are important characteristics to be considered in the present application. The sensitivity of the CCD is the ability to accumulate the signal charges which correspond to the quantity of incident light. On the other hand, the CCD contains noise charges caused by dark current and smearing. Therefore, the signal charges for the part of the received image with low brightness are buried under the noise charges, resulting in an unclear picture image. The dynamic range of the CCD is determined by the amount of the noise charges.
Some unclear picture images which are displayed are attributable to the spectral luminous efficiency. That is, in a method in which 3 primary color light are used, the received image formed by the same brightness decreases in brightness on the display screen in the following descending order: green, red, blue. Therefore, in an endoscope using this method, accurate level control cannot be expected if the control is made in accordance with the brightness level of the 3 primary color signals. That is, it is necessary to change the control sensitivity in the order of blue, red and green.
As an example of an endoscope which addresses the aforementioned problems, the one shown in FIG. 1, is known in the art. The endoscope shown in FIG. 1 uses a color imaging method with a color filter.
In FIG. 1, the entire endoscope is indicated by reference numeral 1, and in the rigid front end portion of the inserting part 2 an imaging means is provided.
That is, at the opening formed in the front end portion, an object lens 3 is positioned to form an image at a specified focal plane. CCD 4 is positioned such that its imaging surface (light receiving surface) is positioned at the focal plane of said object lens 3.
An analyzer (polarizer) 5 is positioned in the optical path between object lens 3 and CCD 4, for example, at the pupil position of object lens 3. Thus, the reflected light from a subject is incident on the imaging surface through analyzer 5. Analyzer 5 can be rotated through an angular range of about 90 degrees by a driver 6.
The subject image formed on the aforementioned imaging surface is converted into video signals and input to TV signal converter 7. The video signals are then converted into 3 RGB color TV signals by TV signal converter 7 and applied to the RGB terminals of the color TV monitor 8 for display.
In addition to the aforementioned imaging means, the inserting part 2 includes an illuminating means. A light guide 9 is formed of an optical fiber bundle, the rear end of which is detachably connected to a light source device 10. Light source device 10 has an illuminating lamp 11 for emitting light which is reflected by the concave surface of a reflecting mirror 12 and condensed by a condenser 13 to be projected onto the rear end of light guide 9.
The illuminating light condensed by condenser 13 passes through light guide 9, and is emitted at the front end face of the light guide 9. The light is then projected onto a light distributing lens 14 and expanded.
A polarizer 15 is positioned at a converged position of the light distributing lens 14, between the light distributing lens 14 and the front end face of the light guide 9, to polarize the illuminating light projected from the front end face of the light guide 9 into a polarized wave (e.g. P wave).
Polarizer 15 disposed on the side of the illuminating means and analyzer 5 on the side of the imaging means form a portion of the means for changing the quantity of illuminating light by detecting the output signal level according to the change of the quantity of illuminating light and adjusting analyzer 5 accordingly.
The video signal output from the CCD 4 is applied to one input of comparator 17. The other input of comparator 7 receives a voltage level set at a reference level Vs such as a saturation level. When the video signal is higher than the reference level Vs, a high level signal is output from comparator 17. The output of the comparator 17 is integrated for a 1 frame period by integrator 18 and the integrated value is then amplified by the amplifier 19 and subtracted from a level Vo where the integrated value corresponds to 0 by subtracter 20 to control the driver 6. Thus a regression circuit is formed.
The driving force of the driver 6 is changed in accordance with the control signal (driving signal) level input through subtracter 20. For example, as in a moving coil-type voltmeter, if the input signal level is high, the driving force is increased against a helical spring, etc., an initializing force, and the analyzer 5 is installed on its rotating shaft (which is set to pass only the P wave component when not rotated) is rotated through an angle in accordance with the level of the control signal.
On the front surface of the imaging surface of CCD 4, a 3 primary color filter of, for example, the mosaic type is provided. The light receiving elements of the imaging surface receive light which corresponds to the picture elements of each wavelength of the 3 primary colors. The picture elements are separated into each primary color signal by the sample hold circuit within the TV signal converter 7. After the horizontal synchronizing signal and vertical synchronizing signal are superimposed, they are output as RGB color signals.
In the example shown in FIG. 1, the driver 6 is driven by the control signal from the regression circuit to change the rotational angle of analyzer 5. In accordance with the rotational angle of analyzer 5, the quantity of light incident on the imaging surface can be controlled.
In the aforementioned example, however, the quantity of light incident on CCD 4 is simply decreased or increased in accordance with the change of the output signal level. It does not provide such a control in which uniformly clear picture images can be obtained from a small quantity of light or a large quantity of light. Although, it may prevent, to some extent, the dynamic range from decreasing.
The aforementioned example also has a disadvantage in that it takes a long time from when then output signal level is changed to the time when the analyzer 5 is rotated in the specified angle.
Furthermore, in a conventional endoscope, the quantity of light is adjusted by manually switching the ND (neutral density) filter inserted in the beam in the light source device. Therefore, it also lacks the quick response and has the disadvantage that the light quantity adjustment can only be made in several stages.